1. Field of the Invention
This invention relates to a magnetic resonance device, for use in magnetic resonance imaging (MRI) systems, in particular a device of the type having a compliant seal and acoustic barrier.
2. Description of the Prior Art
A problem with MRI systems is that noise is generated by the gradient coils of the MRI scanners. The current applied to gradient coils within a gradient coil assembly during an MRI excitation results in vibration of the gradient coil assembly. During an imaging cycle of an MRI scanner the gradient coils are pulsed with high current at frequencies up to 3 kHz. This sequencing creates large non-symmetrical internal forces within the gradient coil assembly as well as an alternating stray electromagnetic field that penetrates outwardly into the outer vacuum chamber (OVC) structure and beyond. These phenomena result in direct mechanical vibrations of the gradient coil assembly that are transferred into the bore tube of the OVC, as well as the formation of eddy currents within the surrounding conductive material that stimulate magnetic field interferences and electromagnetically induced vibration within these conductive structures. Noise generated is coupled to the outside world through the mechanical contact between the gradient coil and the OVC bore tube, as well as by acoustic transmission through the air gap between the gradient coil and the OVC bore tube. This gap is the result of the required clearance to allow mounting of the gradient coil
The quantifiable effects of the vibration and eddy current formation are that noise occurs during imaging, causing patient and operator discomfort; the induced eddy currents disturb the magnetic field within the imaging volume causing image ghosting; and gradient stray fields penetrate the OVC and induce eddy currents and vibrations within the thermal shield resulting in helium boil-off.
The manner of fixing of the gradient coil assembly onto the OVC can affect the degree to which the above phenomena occur. Conventionally, twenty four injection molded wedges, twelve at each end, rigidly fix the gradient coil assembly to the OVC bore tube. This rigid fixing provides a path for the mechanical vibration to pass into the surrounding OVC structure, thus increasing the noise and vibration of the system and also creating secondary induced eddy currents within the material due to its movement within the strong magnetic environment.
DE 10 2004 047344 provides sound and vibration isolation using an elastomeric seal. A sound and vibration damping former is made of fiber layers around an elastomer foil and gaps are provided in the elastomer foil as a passageway for not yet hardened poured resin.
JP2005245775 describes a gradient coil mounting system consisting of liquid filled pads, whose viscosity can be actively controlled to adjust the spring constant and the viscous damping co-efficient according to a sequence.
GB2395279 describes using cushions dispersed around and stuck onto movable gradient coil units, to fix, as well as to provide vibration damping for the movable units, by pumping up the cushions, which can be filled with gas, liquid or foam.